The present invention relates generally to a method for increasing energy confinement and controlling transport in the plasma of a tokamak. More particularly, the present invention relates to a method of creating and sustaining a radial electric field throughout a substantial portion of the cross-section of the plasma by injecting electrons from an external source into the plasma.
The apparatus for toroidal magnetic confinement that is most popular in controlled fusion research today is the tokamak device. To date, the experiments that have been performed with tokamaks to create high temperature plasmas have been of short duration. The duration and energy confinement of the tokamak plasma must be increased to produce useful amounts of energy with this device. A radial electric field within the plasma of a tokamak has been experimentally shown to increase energy and particle confinement. Theoretical work has been the basis of the proposition that radial electric fields can reduce particle and energy loss from tokamak plasmas. Experimental results reported by Oren et al, J. of Nuclear Materials 111 & 112, 34, (1982) demonstrated that overall confinement time would increase by a factor of 10 when a radial electric field was created in the plasma of a tokamak by a cold cathode or a tungsten filament. Particle and energy confinement are known to be closely related. The radial electric field that was created in the plasma by this method was of significant magnitude only at the extreme edges of the plasma. A problem which arises from longer duration plasmas is the accumulation of impurities and if fusion is occurring, fusion waste products such as helium ash. Impurity accumulation within the plasma was demonstrated by Oren et al. when a negative potential was induced within the plasma. The accumulation approached a constant value as the radial electric field decreased.
M. Ono et al., Phys. Rev. Letters 60, 294 (1988) reported improvements in both energy and particle confinement times associated with radial electric fields produced by radiofrequency heating of a tokamak plasma. The magnitude of internal turbulence was observed to be greatly reduced in the presence of the radial electric field, and energy confinement times improved by 30%.
Itoh and Itoh, Phys. Rev. Letters 60, 2276 (1988) and Shaing et al., Comments Plasma Phys. Controlled Fusion 12, 69 (1988) present theories which claim that radial electric fields will influence the transport of particles in tokamak plasmas and will thus affect confinement of both particles and energy. The method postulated is that a resonant interaction between the magnetic structure and particles of a particular velocity causes rapid transport of those particles to the edge of the plasma. The application of a radial electric field shifts the resonant velocity from one that many particles possess to one shared by only a few. Hence, only a few particles then participate in transport. In particular, Shaing et al. suggest that producing a negative radial electric field will reduce transport and improve confinement.
Recent work by R. J. Taylor et al., Phys. Rev. Letters, 63, 2365 (1989) suggest that radial electric fields in tokamak plasmas can have a substantial beneficial effect upon energy and particle confinement. In particular, Taylor et. al's biasing of the plasma center negative with respect to the wall seemed to reproduce many features of the "H-mode" regime of plasma confinement. This was demonstrated by inserting a material electrode into a tokamak plasma and applying a negative bias. For higher temperature plasmas, experimentation with radial electric fields would require a non-invasive biasing technique.
The transition of a plasma into the H-mode is marked by a sudden decrease in the hydrogenic light emission from the plasma edge, followed by a prolonged increase in the plasma density. The reduction of hydrogen light (H.sub..alpha. or H.sub..beta.) indicates that the incoming neutral particle flux is reduced, presumably because of a decrease of the outgoing plasma flux, leading to a reduction in "recycling." The improvement in the energy confinement is generally less than the increase in particle confinement. H-mode measurements also reveal the formation of sharp density and temperature gradients inside the last closed magnetic surfaces, which represents a transport barrier. Despite the magnitude of the effort aimed at modeling the H-mode, no clear mechanism has been identified, although radial electric fields are thought to play a role.
Accordingly, it is an object of the present invention to provide a method and apparatus for controlling transport in a tokamak plasma.
Another object of the present invention is to provide a method and apparatus for stabilizing highly perpendicular velocity electrons against kinetic instabilities in a tokamak.
A further object of the present invention is to provide a non-invasive biasing technique for creating radial electric fields in a tokamak.
Yet a further object is to provide an arrangement for trapping electrons in the interior of a tokamak plasma and charging it negative with respect to the edge to improve confinement properties.